Talk:Poynting vector

Magnetic field naming

This page had the naming of the B and H field wrong in a couple locations. I changed them so that they are consistent with both the rest of the page and the magnetic field page. Hopefully this will reduce confusion of their naming in the future.

W is not defined in the coax section

The variable W has never been defined in the coax section and used without definition of what it is. — Preceding unsigned comment added by Mishapom (talk • contribs) 18:42, 23 January 2025 (UTC)

It is defined three lines further down. It is just a convenient intermediate variable with the dimensions of volts. Constant314 (talk) 19:48, 23 January 2025 (UTC)

What is "direction areal power density"?

Definition section says: "In simple terms, the Poynting vector S gives the direction areal power density that are due to electromagnetic fields in a region of space."

But nowhere in this article does it define (or at least link to) what "areal" means. Could "direction areal" possibly just be "directional" misspelled? Em3rgent0rdr (talk) 04:55, 26 July 2025 (UTC)

Another possibility: maybe "areal" could have been referring to "real power" as opposed to "reactive power"? Em3rgent0rdr (talk) 05:00, 26 July 2025 (UTC)

I think it should be "of real". Constant314 (talk) 06:28, 26 July 2025 (UTC)

OK. Reading the sentence again and I'm thinking it really needs to also include "magnitude" (because a vector has both a magnitude in addition to a direction) and "flow" to be more like:

"In simple terms, the Poynting vector S gives the direction and magnitude of real power density flow due to electromagnetic fields in a region of space."

I also notice the link real power points to a section in AC power but I think a quick sentence summary on what is meant by real power might be needed after this sentence too. I'll let you or anyone else familiar with Poynting vectors make the edit since I'm not. Em3rgent0rdr (talk) 15:59, 29 July 2025 (UTC)

Shouldn't first sentence specify "at a point"?

The intro says "the Poynting vector" (singular) "represents the directional energy flux ... or power flow of an electromagnetic field."

But more precisely, a single Poynting vector just represents this power flow "at **a particular point** of an electromagnetic field", not for the entirety of an electromagnetic field (which is a vector field that exists across all over space & time). (If we were to talk about the whole EM field, then I think we would have to say something more like "the vector field of Poynting vectors" represents the power flow "of an electromagnetic field", but that is too wordy.) Em3rgent0rdr (talk) 03:17, 30 July 2025 (UTC)

The Poynting vector at a point represents the power density at that point. But that is the default meaning of any function of any space. Constant314 (talk) 03:25, 30 July 2025 (UTC)

I guess I should elaborate. In Poynting's original analysis, S = E x H is just a vector such that if you compute the surface integral of that vector on the closed surface that bounds a region, then you get the total energy leaving or entering that region. It is tempting to assume then that the Poynting vector at a point is the power flow at that point, you cannot prove it from Poynting's theorem. However, physicists, being pragmatic, just go ahead and say that it is true.

Subsequent developments allowed for the conclusion that the Poynting vector at a point must be the power density at that point. Sorry, but I cannot recall what those subsequent developments were, but I think it was a consequence of the Stress–energy tensor. Constant314 (talk) 03:48, 30 July 2025 (UTC)

Well thanks for the edits emphasizing "at a point" and adding "magnitude" and correcting the "areal" typo, so I'll consider my quibbles to be resolved. Em3rgent0rdr (talk) 06:29, 30 July 2025 (UTC)

Alleged consequence

One consequence of the Poynting formula is that for the electromagnetic field to do work, both magnetic and electric fields must be present. The magnetic field alone or the electric field alone cannot do any work.

The citation given for this is no longer available, I couldn't find an archived version either so it's unclear what it says exactly. Or even what that "formula" is supposed to be, we could about guess it, but that's not great obviously. As it stands though the claim is in need of backup, with all due respect it doesn't make huge sense to me. 2001:9E8:6ABF:7600:41F:982A:9742:554E (talk) 23:21, 23 August 2025 (UTC)

The formula is P=ExH. Obviously is either E or H is zero then P=0. Constant314 (talk) 23:44, 23 August 2025 (UTC)